John,
That dissimilar combination is common. My thought is, that if you anticipate the carbon steel material will hold up under the corrosive medium then so will the weld if you are using a 309 filler, which is commonly applied to these types of dissimilars. In fact, if the corrosive medium is applied to the cap of a multiple pass groove or fillet application using 309 you will probably have higher alloy content in the 309 cap beads  than you would in the original 304 BM, unless you expereince excessive dilution.

Hello John, I would only make one other comment to the information that js55 gave you in this regard. If these hold-downs are going to be subjected to outside elements, chemicals, or moisture, it is likely that you will be painting or protecting them in some fashion. If that is the case you may want to consider seal welding them at any points where they are welded to the stainless steel tank. This will aid in preventing any sort of leach corrosion between the two parts. Just a thought for consideration. Regards, aevald